.\" Copyright (C) 2022 Jens Axboe <axboe@kernel.dk>
.\"
.\" SPDX-License-Identifier: LGPL-2.0-or-later
.\"
.TH io_uring_prep_send_zc 3 "September 6, 2022" "liburing-2.3" "liburing Manual"
.SH NAME
io_uring_prep_send_zc \- prepare a zerocopy send request
.SH SYNOPSIS
.nf
.B #include <liburing.h>
.PP
.BI "void io_uring_prep_send_zc(struct io_uring_sqe *" sqe ","
.BI "                           int " sockfd ","
.BI "                           const void *" buf ","
.BI "                           size_t " len ","
.BI "                           int " flags ","
.BI "                           unsigned " zc_flags ");"
.PP
.BI "void io_uring_prep_send_zc_fixed(struct io_uring_sqe *" sqe ","
.BI "                                 int " sockfd ","
.BI "                                 const void *" buf ","
.BI "                                 size_t " len ","
.BI "                                 int " flags ","
.BI "                                 unsigned " zc_flags ");"
.BI "                                 unsigned " buf_index ");"
.fi
.SH DESCRIPTION
.PP
The
.BR io_uring_prep_send_zc (3)
function prepares a zerocopy send request. The submission queue entry
.I sqe
is setup to use the file descriptor
.I sockfd
to start sending the data from
.I buf
of size
.I len
bytes with send modifier flags
.I flags
and zerocopy modifier flags
.IR zc_flags .

The 
.BR io_uring_prep_send_zc_fixed (3)
works just like
.BR io_uring_prep_send_zc (3)
except it requires the use of buffers that have been registered with 
.BR io_uring_register_buffers (3).
The
.I buf
and
.I len
arguments must fall within a region specified by
.I buf_index
in the previously registered buffer. The buffer need not be aligned with the 
start of the registered buffer.

See
.BR io_uring_prep_send (3)
for a description of flags that can be set in the SQE
.I ioprio
field. In addition to those, the zero-copy send also supports setting
.B IORING_SEND_ZC_REPORT_USAGE .
If set, the notification CQE
.I res
field will report the number of bytes that were copied rather than sent with
zero copy. A value of
.B 0
indicates success. If the value is
.B IORING_NOTIF_USAGE_ZC_COPIED ,
then data was copied.

As opposed to non-zerocopy send requests, a zerocopy send will usually
generate two CQEs. The first CQE holds the result of the send operation itself,
and if that CQE has
.B IORING_CQE_F_MORE
set in the CQE
.I flags
field, then a second notification CQE will be posted for the operation. This
second notification tells the application that the memory associated with the
send is safe to get reused. The second CQE will have
.B IORING_CQE_F_NOTIF
set in the CQE
.I flags
field. Also see the
.BR io_uring_enter (2)
man page for a fuller description of the notification CQE.

Note that using
.B IOSQE_IO_LINK
with this request type requires the setting of
.B MSG_WAITALL
in the
.I flags
argument, as a short send isn't considered an error condition without
that being set.

These functions prepare an async zerocopy
.BR send (2)
request. See that man page for details. For details on the zerocopy nature
of it, see
.BR io_uring_enter (2) .

.SH RETURN VALUE
None
.SH ERRORS
The CQE
.I res
field will contain the result of the operation. See the related man page for
details on possible values. Note that where synchronous system calls will return
.B -1
on failure and set
.I errno
to the actual error value, io_uring never uses
.IR errno .
Instead it returns the negated
.I errno
directly in the CQE
.I res
field. Some common error cases are:
.TP
.B -ENOMEM
The
.BR ulimit (1)
-l setting is too low to support the size of the attempted zero copy send.
Increasing the limit may help
.TP
.B -ENOMEM
The kernel ran out of memory.
.P
.SH SEE ALSO
.BR io_uring_get_sqe (3),
.BR io_uring_submit (3),
.BR io_uring_prep_send (3),
.BR io_uring_enter (2),
.BR send (2)
